Numerous methods are known for the practice of electrophotography. At a general level, these are methods in which a copied article is obtained by an image-forming procedure having a charging step in which an electrostatic latent image-bearing member is charged; an electrostatic latent image-forming step in which an electrostatic latent image is formed on the charged electrostatic latent image-bearing member; a step in which the electrostatic latent image is developed by magnetic toner carried on a magnetic toner-carrying member in order to form a magnetic toner image on the electrostatic latent image-bearing member; a transfer step in which this toner image on the electrostatic latent image-bearing member is transferred to a transfer material; a fixing step in which this toner image is fixed on the recording medium by, for example, the application of heat or pressure; and a cleaning step in which the magnetic toner on the electrostatic latent image-bearing member is removed by a cleaning blade. Copiers and printers are examples of such image-forming apparatuses.
Image-forming apparatuses, e.g., copiers and printers, have in recent years been experiencing increasing diversification in their intended applications and use environments as well as demand for additional improvements in speed, image quality, and stability. For example, printers, which previously were used mainly in office environments, have also entered into use in severe environments, and the generation of stable images even under these circumstances has become critical.
Copiers and printers are also undergoing device downsizing and enhancements in energy efficiency, and magnetic monocomponent development systems that use a favorable magnetic toner are preferably used in this context.
In a magnetic monocomponent development system, development is carried out by transporting a magnetic toner into the development zone using a toner-carrying member (referred to below as a developing sleeve) that incorporates in its interior means of generating a magnetic field, e.g., a magnet roll. In addition, charge is imparted to the magnetic toner mainly by triboelectric charging brought about by rubbing between the magnetic toner and a triboelectric charge-providing member, for example, the developing sleeve. Reducing the size of the developing sleeve is an important technology in particular from the standpoint of reducing the size of the device.
When, for example, the coverage of the magnetic toner by an external additive is inadequate or the magnetic toner is used in a severe environment, e.g., a high-temperature, high-humidity environment (in the following, a severe environment refers to conditions of 40° C. and 95% RH), its triboelectric charging may not proceed uniformly and charging of the magnetic toner may then become nonuniform.
As a result, a phenomenon can occur in which only a portion of the magnetic toner is excessively charged, so-called charge-up, and various image defects may then occur.
In particular, when the developing sleeve has been downsized as referenced above, the development zone of the development nip region is narrowed and the flight of the magnetic toner from the developing sleeve is made more difficult. As a consequence, a portion of the magnetic toner is prone to remain on the developing sleeve and a trend of greater charging instability sets in.
For example, a reduction in image density can occur when charged-up toner remains on the developing sleeve, while an image defect such as fogging in the nonimage areas can be caused when the toner charge is nonuniform. Furthermore, when used after standing for a while in a severe environment, the aggregative behavior exhibited by the toner is increased due to the pressure on the toner in the developer container. In addition, a phenomenon has occurred in which only a portion of the magnetic toner on the developing sleeve undergoes excessive charging and a reduced-density phenomenon has been produced.
In response to these problems, numerous techniques have been proposed in which stabilization of the changes in developing performance and transfer performance that accompany environmental variations is brought about by the addition of strontium titanate—as an external additive that imparts abrasiveness to the magnetic toner in order to prevent residence of the toner at the developing sleeve and as an agent that relaxes the charging performance during development and transfer in order to inhibit charge up.
For example, in Patent Document 1, the attempt is made to lower the variation in charging performance that accompanies environmental variations: this is done through the addition of a complex oxide composed of strontium titanate, strontium carbonate, or titanic acid because this can impart abrasiveness to the magnetic toner.
A certain effect on image problems such as, e.g., charging roller contamination due to faulty cleaning, is in fact obtained under certain prescribed conditions. However, the flowability and aggregative behavior immediately after standing in a severe environment of higher temperature and higher humidity are in particular not adequately addressed, and there is still room for improvement with regard to the reduced initial density after standing in a severe environment. There is room for improvement with these problems in particular when a small-diameter developing sleeve is installed since aggregation of the magnetic toner on the developing sleeve causes the developing performance to deteriorate.
In Patent Document 2, a toner is disclosed for which charge up is inhibited through a lowering of the number of times of toner-to-toner contact; this is achieved by the addition of a strontium titanate whose volumetric particle diameter distribution has a shoulder on the large particle diameter side at 300 nm or above.
This control of the strontium titanate particle diameter does in fact provide a certain effect on the developing characteristics, e.g., sleeve ghosting due to charging defects, under certain prescribed conditions. However, the problem of charge up produced due to the detachment of large-diameter strontium titanate particles is not adequately addressed, and there is room for improvement with these problems in particular when a small-diameter developing sleeve is installed since the developing zone is then narrow and the charged-up toner undergoes development with difficulty.
On the other hand, in order to solve the problems associated with external additives, toners have been disclosed with a particular focus on the release of external additives (refer to Patent Documents 3 and 4). The charging stability of magnetic toners is again not adequately addressed in these cases.
Moreover, Patent Document 5 teaches stabilization of the development•transfer steps by controlling the total coverage ratio of the toner base particles by the external additives, and a certain effect is in fact obtained by controlling the theoretical coverage ratio, provided by calculation, for a certain prescribed toner base particle. However, the actual state of binding by external additives may be substantially different from the value calculated assuming the toner to be a sphere, and, for magnetic toners in particular, achieving the effects of the present invention without controlling the actual state of external additive binding has proven to be entirely unsatisfactory.